Low-temperature combustion and apparatus therefor



Patented May 30, 1950 LOW-TEMPERATURE COMBUSTION AND APPARATUS THEREFOR Charles E. Hemminger, Westfield, N. J., assignor to Standard Oil Development Company, a corporation of Delaware Application September 26, 1944, Serial No. 555,807

3 Claims.

The present invention is fully disclosed in the following specification and claims considered in connection with the accompanying drawing which'illustrates a preferred embodiment of the novel features of the said invention.

The object of the invention is to provide an efiicient apparatus and process in which powdered coal may be burned under conditions such that the maximum quantity of heat is recovered and contained in formed steam by burning a given quantity of coal in a water tube steam boiler of improved design.

When powdered coal is burned in a firebox of an ordinary steam boiler, heat transfer is effected by radiation to the boiler tubes disposed immediately above the firebox and by convection when the hot gases of combustion 4138.55 through and over the bank of tubes containing water. Because the greater part of the heat load is in vaporization of the water, with high heat transfer coefficient on the inside walls of the tubes, the principal factor governing the tube surface requirements is the outside surface coefficient of heat transfer, which is rather low even with clean tubes. Since the outer tube surfaces do not remain clean because of the slagging action of the ash which deposits on the tubes, optimum results from the standpoint of heat transfer are not obtainable in this type of installation.

Also in the ordinary type of steam boilers using powdered coal as fuel, the slag in the coal is more or less acidic and therefore reacts chemically with the furnace lining causing the conversion thereof into a liquid mass which causes serious loss of said lining and requires frequent repair thereof. In the case of coals forming high melting slags the refractory linings may be protected by disposing tubes containing flowing water in close proximity thereto, thus cooling the same and protecting them against the corrosive action of the slag. In the use of low melting slags, another proposal for protecting the furnace lining is to use a high melting point and specially prepared refractory material as the furnace lining. It can be seen that which ever expedient is resorted to there is involved additional and/or expensive supplementary equipment and/or material. In the case where cooling tubes are disposed in close proximity, the efllciency of vaporizing the water therein is necessarily poor due to poor gasiform material circulation in this region. Also where high melting expensive refractory is used to line the furnace the molten slag is drawn off, as in a blast furnace, thus of course involving additional equip- 2 ment, in addition to the greatercost of the lining.

According to my present process, I cause combustion of the powdered coal at a sufficiently low temperature that there is substantially no fusing and consequently a minimum amount or no slag, in a flexible operation affording maximum mixing of powdered coal and air, and conversion of coal to fly ash and flue gases almost completely. By fly ash I mean unfused .incombustibles, in more or less finely divided form produced by burning coal containing ash constituents.

In the accompanying drawing, I have shown diagrammatically a preferred modification of a suitable apparatus embodying my improvements.

Referring in detail to the drawing, I provide a combustion chamber I and a heat recovery chamher 2 disposed above the combustion boiler and in communication therewith as-shown. In operating the device, air enters the system through line 3 into which I discharge from a suitable powdered coal supply hopper 5 through a star feeder I and a standpipe I0 a quantity of powdered coal having a particle size of from 50 to 400 mesh, preferably about 200 mesh. The standpipe is provided with a number of taps l2 into which I may force a slow current of gaslform material, such as air, to prevent bridging and plugging in the standpipe. In line 3 the coal is formed into a suspension which I then convey into the bottom of combustion chamber I and cause the same to flow upwardly therein under delayed. settling conditions so as to form a dense, turbulent, suspension of powdered coal in air. I term the coal in this state to be fluidized. I accomplish this by controlling the velocity of the air in the combustion zone I within the limits of from /2 to 5 ft. per second, preferably 1% to 3 ft. per second, whereupon I form the dense suspension referred to, which will have a weight or density of from about 3 to 25 lbs. per cubic foot. Also by controlling the amount of powdered coal per unit of air fed to the combustion vessel I, I may fix the upper level L of the dense phase suspension at some desired height above which the concentration of powdered material in the gas is greatly reduced, so that as the material issues through line 20 the concentration of suspended solids may be of the order of 0.001 lb. per cubic foot, more or less.

Referring back to the combustion zone I, the

dense phase suspension of coal in air undergoes combustion and the temperature is maintained within the limits of, say, 1000 to 2200" F. with 15001'700 F. preferred, at which temperature, while there is ood burning of the coal, the temperature is insufllciently high to permit fusion of the mass and therefore, there is a minimum of slagging.

The ratio of coal fed to the combustion zone with respect to air is of the order of 0.004 to 0.007 lb. powdered coal per cubic foot of air measured under standard conditions of temperature and pressure.

In the combustion zone I, I dispose a plurality of water tubes t. In the drawing I have shown four, but actually there may be a large number of these tubes disposed in the combustion zone. Water is fedthrough a pipe 30 into a manifold 32 and thereafter passes into the tubes t, the steam being withdrawn from a steam manifold 35. Due to the fact that the coal is consumed by combustion to form essentially flue gas and fly ash, with substantially no slagging, there are virtually no deposits on the walls of tubes t, and therefore the same are maintained in a clean condition. This, of course, accentuates the transfer of heat from outside the tubes through the wall to the interior, also the heat transfer to the tubes is also accentuated by-the constant contact of the hot coal particles against the walls of th tube to break down the stagnant gas film thereupon. Uniformity of temperature in the combustion zone is also maintained due to the turbulent action of the mass of powdered coal in air. The suspension coming into combustion chamber I passes through a grid G which serves to distribute the suspension uniformly into the space between grid G and the upper level L, and the distribution and motion or mobility of the particles of coal makes it possible to so operate the proces that in spite of the fact'that a large number of B. t. u.s per hour are liberated in the combustion chamber, no two points differ in tem-' perature more than a few degrees F., say a maximum of 15 or so. The products of combustion issue from combustion chamber I via line 20 and pass into zone 2, where further burning occurs.

The main pointv or one of the main points of the invention, as previously indicated, is to cause the combustion of the coal at a sufliciently low temperature to prevent substantial fusion of the coal. I accomplish this by limiting the height of the coal .in dense phase suspension to say 15 to 22 ft. above grid G, preferably about 20 ft. (from G to L in reactor I). For each pound of coal,--12 lbs. of steam are generated. .The density of the suspension is above 3 lbs. per cubic foot preferably around lbs. per cubic foot. Now when using 0.004 to 0.007 lb. of coal per cubic foot of air measured under standard conditions (and observing the other conditions noted), the temperature of the combustion may be maintained below 2200 F. and preferably is maintained between about 1500 F. and 1700 F. Uniformity of temperature and clean tubes will be maintained by avoidance of slagging and the constant turbulent state of the fluidized solids in the suspension.

The gases issuing from combustion zone I contain combustibles and fly ash" and a secondary quantity of air is introduced through line 30 into line and passes with the gases and/or solids entrained therein through a grid G into the body of the heat recovery section 2. In this heat recovery section there is disposed a second bank of tubes t similar to those in combustion zone I, and as before, water is introduced through manifold 32a, heated in tubes t and withdrawn as steam through manifold 35--a. The flow of gases and fumes through the heat recovery section 2 is controlled within the limits of 5 ft. per second, preferably 1% to 3 ft. per second, so as to form a dense suspension having an upper level at L, in the same manner as was accomplished in the combustion zone I. Here also due to the motion and mobility of the particles of ash, coke, etc. forming the solid material in the dense suspension there is uniformity of temperature between the grid G' and the upper level L, and the tubes are maintained in clean condition due to the fact that there is no deposition of slag or ash.

The purpose of the secondary heat recovery section is to reduce the temperature of the flue gases and to consume unburnt combustibles as carbon monoxide, hydrogen, methane and coal. The secondary section is usually operated at 200 to 1000 F. lower temperature than the primary section. Due to the depletion of carbon the concentration of fly ash or inorganic incombustible in the second-section is very high, If desired, circulation of solids between the two sections may be caused b flow through standpipe 4| by adjusting valve 42. Also much of the fly ash in the secondary section may be removed from the system through line 43. As indicated later this secondary section may be omitted.

The gases and fumes issue from the zone 2 through line 50, pass through an air preheating zone 52 and are finally vented from the system along with suspended fly ash through line 55. The air which passes into the heat exchanger or economizer 52 via line 3---a and exits through line 3-1) is of course delivered to line 3 for use in the system and another portion passes through line 3-0 to secondary air inlet 20. The fly ash may be removed from the air in line 55 by known means such as centrifugal or electrical separating devices.

In a modification of my invention, the chamber 52 may take the form of a conventional waste heat boiler where the sensible heat of the gases is recovered in the form of steam or in the form of preheated water to feed the other two sections of the boiler. Then it may be desirable to eliminate the secondary section 2 and pass from section I to the waste heat boiler. However, it is also desirable to feed air from line 3 -c into line 20 to burn all of the unconsumed combustibles before or in the waste heat boiler.

Other fuels may be burnt in the system. Any

- type of solid fuel may be fed into hopper 5. Also,

gaseous or liquid fuels may be fed to section 2 through line I4.

Obviously, other materials as mercury, gases. oil or heat transfer fluid may be heated in the tubes of the several sections. The invention is not limited to coal as 50 mesh or smaller. In

- fact, coal of up to size (4 mesh) may be used,

in which case the velocity to .obtain the even flow and agitation in the bed in I must be say 3 to 7 ft. per-second. A preferred operation is however with /8"0 size (8 mesh) coal with about 3 ft./sec. air velocity in'zone I. When the larger coal is used, it is desirable to have a low velocity in the secondary section 2, in which oniythe smaller, unburnt coal will be present. Here the air velocity will be less than 3 ft./sec., say 1% to 2 ft. per second. With experience the intelligent worker will be enabled to adjust gas velocities with respect to particle size in zones l and 2, as indicated herein, to obtain the desired state of fluidized solids or dense suspension in zone I and 2 for sufficient information has been given to attain these results, although every possible condition oi. particle size versus gas velocity has not been specifically mentioned.

Numerous modifications of my invention will appear to those who are familiar with this art.

What I claim is:

1. In a method of burning a solid fuel to form steam in a two-stage process in which water is converted to steam by heat exchange with the burning fuel, the improvement which comprises causing the fuel in powdered condition to be fluidized by passing upwardly through a body of said fuel in a first combustion zone, a continuous quantity of air moving at a velocity of from /g to 5 ft. per second, maintaining elevated temperatures in said combustion zone whereby burning of the fuel is effected, controlling temperature conditions within the said zone at a sufiiciently low level that there is substantially no fusing oi the fuel, providing a plurality of confined flowing streams of water in heat transfer relationship with, the said fuel undergoing burning whereby the water is converted into steam by the heat evolved during the burning of the said fuel, recovering said steam. causing the fumes of combustion, together with solids including unburned solid fuel, ash and coke, to pass upwardly from the combustion zone into a second combustion zone, adding an additional quantity of air to said fumes and said solids, causing the mixture to pass upwardly through said second combustion zone to form therein a second dense suspension of fluidized solids in said fumes and air, causing a plurality of confined streams of water to flow within the said second combustion zone in heat exchange relationship with the solids and the gaseous combustion products formed therein, whereby additional steam is formed and recovering a second quantity of steam from said second combustion zone.

2. The method of claim 1 in which the temperature in the first combustion zone is maintained between about 1000" F. and 2200 F. by limitin the fuel fed to the combustion zone to from about 0.004 to 0.007 lb. per cubic foot of oxygen-containing gas measured at standard conditions.

3. The method of claim 1 in which the temperature in the first combustion zone is maintained within the limits of 1500 and 1700 F. by controlling the height of the dense suspension.

CHARLES E. HEMMINGE'R.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

